1. Field of the Invention
The present invention relates to a level shifter which is applicable to a power amplification system, and more particularly, to a level shifter in which a plurality of amplifiers are connected in parallel, whereby a forward isolation spec of a power amplifier can be improved and the level shifter can be relatively scaled down.
2. Description of the Related Art
At present, a great deal of research has been conducted into a power amplifier (PA) using complementary-metal-oxide-semiconductor (CMOS) technology. A power amplifier includes a high frequency block which manages signal transmission and reception, and an analog control block which controls the power amplifier. The analog control block needs to be delicately operated.
In particular, many circuit design technologies are integrated into the analog control block in order to meet the operational specification of the power amplifier.
Typically, a level shifter may be used when it is necessary to change an input voltage range to a desired output voltage range.
As for a conventional level shifter, a reference buffer and an input buffer are connected in parallel, and an output amplifier is connected to an output node to which output terminals of the reference buffer and the input buffer are commonly connected.
In such a conventional level shifter, an input voltage (Vin) ranges from 0 V to 0.2 V in a standby mode. In the standby mode, the power amplifier is in an off state. Thus, output power of the power amplifier should meet a forward isolation spec of the power amplifier. That is, the output power of the power amplifier should be lower than a reference power.
To meet the forward isolation spec of the power amplifier, the output voltage (Vout) of the conventional level shifter should be low when the input voltage (Vin) ranges from 0 V to 0.2 V.
In an off period (0-0.2 V) of the power amplifier, the conventional level shifter should meet the forward isolation spec. However, even in a low input voltage (0-0.2 V), the conventional level shifter may generate an output voltage higher than a predetermined voltage. Therefore, the conventional level shifter may not meet the forward isolation spec of the power amplifier.